Polarographic Determination of Traces of Nitrilotriacetate in Water Samples John P. Haberman The Procter & Gamble Company, Miami Valley Labs, Cincinnati, Ohio 45239
A sensitive and selective method for determining trace amounts of nitrilotriacetate, a component in detergent formulations, is described. Polarographic analysis of NTA as an In(lll)-NTA complex was used. The technique was evaluated in river water and sewage samples from 0.0257 to 2.57 ppm with anion exchange concentration and isotope dilution to correct for incomplete recovery. It was evaluated from 2.57 to 257 ppm without anion exchange concentration. The relative standard deviation was approximately lo%, except for the lowest concentration. FEWPRACTICAL APPLICATIONS of electroanalytical techniques to environmental analyses have been reported. In this study, nitrilotriacetate (NTA) in sewage and river water samples was converted to a polarographically active complex with In(II1) in 1 M NaCl, 0.1M acetic acid (HOAc), and 0.1M sodium acetate (NaOAc) buffer. Anion exchange concentration and differential polarography were used to increase the sensitivity. A cation exchange column pretreatment was used as a precaution against interfering cations, and isotope dilution analysis with IT-labeled NTA (NTA-IC) was used to determine the variability of the concentration step. Colorimetric methods for NTA analysis based on the bleaching of colored complexes have been described ( I , 2). Although sensitive and rapid, they are subject to interference by other chelating agents. Colored material in samples may also interfere. Theoretically, other chelating agents could cause polarographic interference (3), but in this study using In(II1) there was no interference noted. EXPERIMENTAL PROCEDURES, MATERIALS AND EQUIPMENT
The following steps constituted the procedure used for the analysis of NTA added to waste water samples. Sample collection and storage were not investigated. Figure 1 is a schematic representation of the complete procedure. Analytical Procedure below 2.57 ppm. INITIALPRETREATMENT. One liter of filtered sample was measured into a beaker and analytical studies were performed by adding known amounts of NTA to samples at this point. An aliquot of NTA-14C was added for isotope dilution analysis and the pH was adjusted to 3.0 by the dropwise addition of 6 M HCl. Nitrogen gas was bubbled through the sample with a gas dispersing tube for 10 minutes at a rate sufficient to give good stirring to remove H2S. CATION EXCHANGE PRETREATMENT. The sample was passed through a 60-ml (approximately 1.8 x 26 cm) column of cation exchange resin. After the sample had passed through the column, it was rinsed with 200 ml of distilled water, and this rinse was added to the sample. The flow rate of the column was adjusted so that approximately 1 hour was required for passing the sample and rinse through the column. The resin was discarded after each sample. (1) J. E. Thompson and J. Duthie, J. Water PoNut. Contr. Fed., 40, Pt. 1,306 (1968).
(2) R. D. Swisher, M. M. Crutchfield, and D. W. Caldwell, Enuiron. Sci. Technol., 1, 820 (1967). (3) W. Hoyle, I. P. Sanderson, and T. S. West, J. Electroanal. Chem., 2, 166 (1961).
ANIONEXCHANGE ABSORPTION. The pH of the sample was adjusted to 7.0 by the dropwise addition of 50% NaOH solution and the sampIe was passed through a 15-ml (approximately 1.4 X 12 cm) column of anion exchange resin. The flow rate was adjusted so that approximately 1 hour was required for the 1.2-liter sample to pass through the column. The column was then rinsed with 100 ml of distilled water, and all column effluent was discarded. ELUTIONOF ANIONEXCHANGE COLUMN.The NTA was stripped from the column with 1 M NaCl in 0.1M NaOAc and O.1M HOAc buffer of pH 4.7. The flow rate was adjusted to approximately 10 ml per minute and the first 10-ml fraction contained most of the distilled water from the free volume of the resin, so it was discarded. The next 10-ml fraction contained approximately half of the NTA and the next 50-ml fraction Contained almost all of the rest of the NTA. The largest concentration factor was obtained by using the second 10-ml fraction for analysis. The anion exchange resin was discarded after each sample. ISOTOPE DILUTION ANALYSIS.A small portion of the fraction of eluant (100 pl) was pipetted directly into a scintillation vial for counting. The recovery of NTA-lC compared to the amount added in the “Initial Pretreatment’’ step was used to calculate the concentration factor. POLAROGRAPHIC ANALYSIS.The rest of the eluant fraction was used for differential polarographic analysis (the eluant served as the supporting electrolyte solution). Three milliliters were pipetted into the sample and reference cells. Both sample and reference solutions were deaerated by bubbling nitrogen gas through the sample in the cells for 5 minutes. A potential of -0.70 volt us. SCE was applied with the current sensitivity at 0.05 pA full scale. In(NO& (0.001M) in 1 M NaCl, 0.1M NaOAc, and 0.1M HOAc (pH 4.7) was added to the sample side of the cell with a 1-ml microburet. After each addition, nitrogen gas was bubbled through the solution for stirring [and removal of dissolved oxygen introduced uia the In(II1) solution] and then stopped. When a polarographic current of 0.005-0.02 pA indicated the presence of excess In(LII), addition was stopped and the volume of In(II1) solution added to the cell was noted. Note that for some sewage samples, it was necessary to finish “titrating” the sample with 0.01M In(NO&. A preliminary potential sweep was manually performed to determine the current sensitivity setting which resulted in the best recorder display. Then a cathodic polarographic sweep was performed from an initial potential of -0.10 volt us. SCE and a polarogram was recorded. The height of the excess In(lI1) wave was used as the base line to determine the height of the In(II1)-NTA wave and the current was measured at the peak of the wave (Figure 2). It was necessary to correct the polarographic result for dilution by the volume of In(II1) solution added. The current was correlated to the solution concentration of NTA as ppm Na3NTA via a standard curve. Analytical Procedure above 2.57 ppm. The “Initial Pretreatment” step was the same except that solutions of 100-ml volume were used and NTA-IdC was not added. All other steps until the “Polarographic Analysis” were deleted. Materials and Equipment. Four types of waste water sample were used; Ohio River water was collected at Cincinnati and used without further treatment. Cincinnati municipal sewage (Gest Street Plant) was also used without further
ANALYTICAL CHEMISTRY, VOL. 43, NO. 1, JANUARY 1971
63
0.06
4NTA-’4c 11 SAMPLE 1 I
Tx fi I 1 1 11
,,ELUANT
PH ADJUSTED
ANION I EXCHANGE COLUMN
j/,
PH ADJUSTED,~\
CATION EXCHANGE COLUMN
~
~
TO 7.0
RADIOCHEMICAL ANALYSIS 0.
6*
,
/
I
0.04-
SAMPLE EFFLUENT
CONCENTRATED 0/’ NTA FRACTION
I nW N T A
e
e
\
6
0.02 -
1, ~xcesS Inm
POL A ROGR A f f l IC ANALYSIS
Figure 1. Schematic representation of complete analysis -0.1
treatment. Synthetic sewage prepared from a glucose nutrient broth and dipotassium phosphate was treated under laboratory conditions by activated sludge (aerobic). Synthetic sewage with peptone, glucose, urea, disodium phosphate, sodium chloride, and beef extract was treated under laboratory conditions in sludge digesters (anaerobic). Samples were suction filtered through two thicknesses of glass filter paper. The unlabeled NTA was Matheson Coleman and Bell reagent grade nitrilotriacetic acid and the NTA14Cwas an Amersham/Searle Corp. custom preparation with one carboxyl 14C per molecule. Dowex 50 W-X8 (50-100 mesh) cation exchange resin in the hydrogen ion form was used throughout. Dowex 1-X8 (50-100 mesh) anion exchange resin was used for early experiments and Dowex 21 K (50-100 mesh) was used for later experiments including evaluation with waste water samples (both in the chloride ion form). Both cation exchange and anion exchange resins were prewashed with two cycles involving an HC1 wash and distilled water rinse, then were rinsed to approximately neutral pH with water. In(II1) solutions were prepared by dissolving a weighed quantity of American Smelting and Refining Co. 99.999 % indium metal with dropwise additions of concentrated HNOa while heating on a steam bath. All other chemicals were reagent grade, and used without further purification. Radioanalytical determinations of NTA-I 4c were performed with a Packard Tri-Carb Model 3375 liquid scintillation spectrometer. The differential Polarograph ( 4 ) was a simple one and was programmed on a versatile electrochemical instrument. Details are not given here since a wide variety of commercial electrochemical instruments are available (5).
DISCUSSION Sample collection and storage were not investigated, but if the NTA-IC were added to unfiltered sample at the time of collection, loss of unknown NTA during sample storage would be automatically corrected for if it was in equilibrium with the NTA-14Cadded. Ion Exchange of NTA Samples. CATIONEXCHANGE REMOVAL OF CATIONS. The pH of samples was adjusted to 3.0 in the initial pretreatment step so that the ionic strength introduced was not appreciable while the pH was low enough to (4) L. Meites, “Polarographic Techniques,” 2nd ed., Interscience Publishers, New York, N. Y.,1965, p 573. (5) G. W. Ewing, J. Chem. Educ., 46,A717 (1969).
64
ANALYTICAL CHEMISTRY, VOL. 43,
-0.5
-0.9
-I.
facilitate the displacement of metal ions from NTA complexes (high ionic strength could interfere with the absorption of NTA during the later anion exchange step). The hydrogen ions liberated from the column when the metal ions were absorbed lowered the pH further, with no increase in the ionic strength (after cation exchange treatment, the pH of most samples was in the range of 1.5 to 2.5). The effectiveness of the cation exchange step in removing metal ions was not determined with actual samples but Zn(I1) and Mg(I1) in the presence of NTA were reduced by a factor of lo5 in distilled water. ANIONEXCHANGE ABSORPTION OF NTA. Figure 3 illustrates the absorption of NTA-14C onto Dowex 1-X8 resin. NTA is a divalent anion in the pH range of 5-8 (6) and a pH of 7.0 was selected for most experiments, but absorption was efficient in the pH range of 5-11. The NTA was taken up by the first part of the resin in spite of the fact that 1 liter of sample was passed through the column in each case. The optimum size of anion exchange column was not determined during this work. Smaller volumes of resin in the anion exchange column required less eluant to elute the NTA and resulted in higher concentrations of NTA (note that this consideration did not apply to the cation exchange pretreatment step). ELUTION OF NTA. Figure 4 shows the elution behavior of NTA with sewage samples. Some variability in the elution of NTA from different types of samples was evident from these studies. One of the reasons for selecting anion exchange as a method of concentrating NTA was to have the possibility of a chromatographic elution to separate interferences (7). NO (6) “Stability Constants of Metal-Ion Complexes,” L. G. Sillen and A . E. Martell, Ed., Chem. SOC.(London), Spec. Publ. No. 17, 1964. (7) C. Davies, R. D. Hartley, and G. J. Lawson, J . Chromarogr., 18, 47 (1965).
NO. 1, JANUARY 1971
I
60A
-i 1 l
8
'
0
t
t
C
40j , k D a;p & c
20
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, i + + -
40b
, B
r
2
'0
10
20
30 40
50
0
IO
20
30
40 50 60
I
Volurre of Eluant ,ml
4
40-
B
1 t i
Figure 4. Elution of NTA from Dowex 21 K with 1M NaCl, 0.1M HOAc, and 0.1M NaOAc (pH 4.7)
I
0
I
t I Distance from Top of Golumn,cm Figure 3.
Absorption of NTA onto Dowex 1-X8
A . 25.7 ppm of Na3NTA in distilled water at pH 11.0; 0.1% of sample in column effluent B . 25.7 ppm Na3NTA with 10-fold molar excess of Pb(I1) in distilled water at pH 5.0; 0.9% of sample in column effluent; absorption was as a Pb(I1) complex C. 25.7 ppm Na3NTA in tap water with 7 grain hardness at pH 7.0; pretreated with Dowex 50 W-X8; 0.7% of sample in column effluent D. 0.257 ppm NTA under same conditions as C. above; L 3%of sample in column effluent (limit of detection of available NTA-14C)
evidence of interference during the polarographic analysis step was found in this study so the practice of using a high concentration of salt to strip the NTA (and presumably other materials) from the column was used. It was found that concentrations of NaCl of 1M (and greater) eluted the NTA as soon as the free volume of the resin was displaced. NTA was efficiently stripped from Dowex 21 K resin in this way even when the resin was visibly darkened from material in sewage samples. Table I is a summary of recoveries observed during an evaluation of the analysis. The first 10-ml fraction contained very little NTA-"C. An average concentration factor of 47 resulted from the second 10-ml fraction and an average concentration factor of 15.5 resulted from a total volume of 60 ml of eluant (not including the first 10-ml fraction). Note that the relative standard deviation (RSD) of the concentration factor in the 60-ml fraction was only 7% while that of the 10-ml fraction was 12%. If it was established in practice that the reproducibility of recovery of a particular fraction was well within the experimental error of the whole procedure, then it would be possible to discontinue the isotope dilution step. Polarographic Analysis of NTA as In(II1)-NTA. Other metal cations were investigated for the polarographic analysis of NTA but In(II1) gave the best results during preliminary studies on sewage samples. Without regard for the actual species involved in the equilibrium system and the reduction reactions, they may be represented as follows:
+ NTA s In(II1)-NTA + 3e- -. In(0) In(1IltNTA + 3eIn(0) + NTA
In(II1)
Excess In(II1)
-f
- 0.61 volt US. SCE - 0.79 Volt GS. SCE
The polarographic wave from excess In(II1) in 0.1M acetic acid and 0.1M sodium acetate (pH 4.7) exhibited a halfwave potential of -0.61 volt US. SCE. The In(II1)-NTA reduction wave was well separated from the excess In(II1)
A . 0.0257 ppm of Na3NTA in distilled water; 89% recovery in 60 ml of eluant B . 0.283 ppm of Na3NTA in untreated municipal sewage; 79% recovery in 60 ml of eluant C. 0.283 ppm Na3NTA in activated sludge treated synthetic sewage; 86 % recovery in 60 ml of eluant D. 0.283 ppm Na3NTA in sludge digester treated synthetic sewage; 87 % recovery in 60 ml of eluant
Table I. Average Recovery of NTA-I4C during Evaluation
Fraction, No., ml 2, 10 3, 50 2 + 3 , 60
Recovery, % 46 47 93
RSD, % 12 15 7
wave and exhibited a n unusual peak shape (8). The halfwave potential was found to be -0.79 volt us. SCE and the peak potential was -0.85 volt in the acetate buffer. Apparently the reduction was quasi-reversible and the electroactive species was In(II1) HNTA -t (9). Polarographic studies of other In(II1) complexes which exhibited peak shaped waves have been reported in the literature ( 1 0 , I I ) . In practice, the peak shape of the waves increased the selectivity of the determination because the peak represented an excellent reference point at which to make the current measurement. Standard curves based o n the peak current were linear in appearance but the ratio of the peak current to the NTA concentration changed slightly with the NTA concentration. The conversion of the NTA to the In(II1) complex was a n extension of a technique which has already been described (3). It was controlled by controlling the amount of excess In(II1) in equilibrium with the complex. This was done by adding In(II1) to the sample cell until the current at -0.70 volt us. SCE indicated the presence of approximately the same amount of excess In(II1) for both calibration standards and unknown samples, Figure 5 illustrates the relatively small effect of the amount of excess In(II1) on the reduction current for In(II1)NTA. The horizontal axis represents the excess In(II1). The vertical axis represents the magnitude of the reduction current of the In(II1)-NTA complex compared to the current when the magnitude of the In(II1) wave was in the range used as standard conditions for analyses (0.005-0.02 FA). The points representing standard conditions for each concentration (8) J. Korvta and I. Kossler. Collectioiz Czech. Chem. Cornmu/?.,15, \
,
241 (19jO). (9) R. Staroscik and K. Webs, Chenz. Anal. (Warsaw), 12, 1275 (1967). (10) A. J. Engel et al., ANAL.CHEM., 37,203 (1965). (11) L. Pospisil and R. DeLevie, J . Electroanal. Chem., 25, 245 (1970).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 1, JANUARY 1971
65
NTA resulted in no interference as was also the case for a pprn Na,NTA
0
I
1
I
1.0
0. I
IO Free In (ID), PPm
100
Figure 5. Effect of excess In(II1) on height of In(II1)-NTA wave are the symbols centered on the 0% error line. The height of the In(II1)-NTA wave was within f10% of standard conditions for a large range of excess In(II1) (0.4 to 100% with 257 pprn Na,NTA). This was probably due to competing equilibrium and kinetic (12) effects. The relative standard deviation of standards was approximately 5% (with and without temperature control of the cell). The lower limit of NTA which could be measured above background currents (including the current from the excess In(II1) wave) was about 0.3 ppm when anion exchange concentration was not used and the measurement was made with a differential polarograph. INTERFERENCE STUDIES.N o evidence of interference was detected in distilled water solutions with a tenfold molar ratio of ethylenediaminetetraacetate (EDTA), ethane-1-hydroxy-1, 1-diphosphonate (EHDP), citrate, maleate, phosphate, carbonate or sulfate to 2.57 ppm Na3NTA. A solution containing EHDP and EDTA, each with a tenfold molar ratio to (12) C. Auerbach, ANAL.CHEM., 30, 1723 (1958).
solution containing all of these compounds, each with a tenfold molar ratio to NTA. Although it was often necessary to add much more In(1II) than was necessary for the NTA alone, before excess In(II1) was detected, the polarograms were not affected and the analytical results were within experimental error of being the same in these studies. Anomalous results were observed when In(II1) was added to a distilled water sample which had been treated with a particular batch of Dowex 21 K resin. The resin had been left in contact with concentrated HC1 for 5 hours during pretreatment. When the resin was retreated with 1M HCI in contact with the resin for a short time, normal results were obtained. Benzyltrimethylammonium chloride was added to NTA in distilled water to represent a possible decomposition product of Dowex 21 K resin, but the anomalous results were not reproduced. The wave shapes for the free In(II1) and In(II1)NTA were normal but the height of the In(II1)-NTA wave was decreased 29% when 200 ppm of benzyltrimethylammonium chloride was present and 41 when 400 ppm was present. In practice, materials of this type which might occur in samples should be efficiently removed during cation exchange treatment. The polarographic reduction of In(II1)-NTA did not appear t o be particularly sensitive to interference by surfactants. Linear alkyl benzene sulfonate concentrations of 3.5 and 43.5 ppm did not affect polarograms and a n analysis for NTA in a sample of a commercial detergent formulation dissolved in electrolyte solution gave a value which was within experimental error of the expected result. EVALUATION. The experiments summarized in Tables I1 and I11 were performed to evaluate the best set of conditions which evolved from this work. One-liter portions of a river water sample, a laboratory activated sludge treated sewage sample, a raw municipal sewage sample, and a laboratory sludge digester-treated sample were carried through the complete procedure with 0.0257, 0.257, and 2.57 ppm of unlabeled NTA added. The second 10-ml fraction of the anion exchange eluant and a third 50-ml fraction were analyzed. The
-
Sample River water
Table 11. Evaluation with Anion Exchange Concentration Analysis of Fractions NTA added, Fraction, NTA-14C, Total NTA, ppm NaaNTA No., ml ppm Na3NTA
z
Error,
z
0.0257 0.0257 0.257 0.257 2.57 2.57
2, 3, 2, 3, 2, 3,
10 50 10 50 10" 50
47 61 56 41 44 55
2.38 0.395 14.5 1.98 11.2 26.2
0.0249 0.0072 0.233 0.216 2.52 2.35
-3 -72 -9 - 16 -2 -9
Activated sludge effluent
0,0257 0.0257 0.257 0.257 2.57 2.57
2, 3, 2, 3, 2, 3,
10 50 10 50 10" 50"
46 45 51 45 52 42
2.02 0.452 12.3 2.35 12.6 2.10
0.0182 0.0245 0.215 0.235 2.39 2.47
- 29 -5 - 16 -9 -7 -4
Raw sewage
0.0257 0.0257 0.257 0.257 2.57 2.57
2, 3, 2, 3, 2, 3,
10 50 10" 50 lo" 50"
36 52 38 51 44 50
1.45 0.585 1.05 2.48 11.8 2.53
0.0146 0.0305 0.250 0.217 2.68 2.50
-43 +I9 -3 - 16 +4 -3
Fraction diluted 10-fold before polarographic analysis.
66
NTA found, ppm Na3NTA
ANALYTICAL CHEMISTRY, VOL. 43, NO. 1, JANUARY 1971
Table 111. Evaluation without Anion Exchange Concentration Error, NTA added, NTA found,
Sample River water
ppm NaSNTA pprn NaaNTA % 2.57 2.49 -3 25.9 25.7 +I 239 -7 257 2.55 Activated sludge 2.57 -1 25.3 -2 effluent 25.7 229 -11 257 2.29 -11 Raw sewage 2.57” 25.2 -2 25.l b -2 252 257b Digester effluent 2.57 2.64 +3 25.7b 31.2 +21 257b 267 $4 Sample diluted 5-fold“or 10-fold6 before polarographic analysis.
accuracy of the analytical results for the two fractions was approximately the same. The relative standard deviation of averaged results was 42, 6, and 5%, respectively, for 0.0257, 0.257, and 2.57 ppm Na2NTA. The analytical results for the digester sample were high but when the NTA added to the samples was subtracted out, a relatively constant blank of 4.0 ppm (13% RSD) was observed. Although contamination was not expected, it was considered more likely that this sample had become contaminated with NTA than that an interference had been encountered. Analytical data from the digester eWuent sample were not included in the results on Table 11. The results of a second series of the same type of samples (collected at a different time) with 2.57, 25.7, and 257 ppm NTA added are presented in Table 111. No NTA-I4C was added and all ion exchange steps were deleted. The relative standard deviation of averaged results was 6, 11, and 7%, respectively, for 2.57, 25.7, and 257 ppm Na3NTA. The results indicate that In(II1) has the ability io displace NTA from other metal ions naturally present in these types of samples without the necessity of cation exchange pretreatment. In this series of experiments, the digester sample did not give anomalously high results and analytical data from the digester sample were included with the other types in Table 111. In both series of experiments, a precipitate sometimes appeared upon addition of In(II1) solution to the sample in the polarographic cell. In order to obtain normal polarographic results, it was then necessary to dilute the sample with electrolyte solution sufficiently that precipitation did not occur. The polarographic residual current was higher than normal at negative potentials with sewage samples and this effect decreased the negative potential range for reductions at a D M E but did not interfere with the reduction of the In(II1)-NTA
complex (see Figure 2). The analytical results at the 0.0257ppm level were marginal but the results at the 0.257-ppm level and greater were much better. The NTA-l*C added for isotope dilution data was 0.0257 ppm (1 mCi/g specific activity) for all of the results with anion exchange concentration. At the 0.0257-ppm level of unlabeled NTA, the presence of the NTA-l4C had the effect of doubling the error in determining the concentration of the unlabeled NTA (which would correspond to the unknown amount in an actual analysis). The use of a smaller amount of a “hotter” sample of N T A - l C would be expected to improve analysis at that level considerably. Before the excess In(II1) wave appeared, it was often necessary to add considerably more In(II1) than was necessary to complex the NTA in the representative samples (see Figure 2 caption). Apparently there were large quantities of material in these samples which formed complexes with In(II1) which were not polarographically active in the electrolyte used. As long as excess In(II1) was present, the presence of polarographically inactive In(II1) complexes did not seem to affect the reduction of the In(II1)-NTA complex. For these studies, differential polarography was used as a more sensitive method than dc polarography. Differential polarography also had the advantage of subtracting out residual currents which might be encountered. In practice, there was no evidence of interfering residual currents in samples and there was no evidence that single-cell polarographic techniques could not be used. There was evidence that organic material in sewage samples interfered with the use of stationary mercury drop electrodes and that techniques employing dropping mercury electrodes were to be preferred. There was no evidence that NTA was lost during the cation exchange step for these studies but large losses were encountered during further development of the method for septic tank samples and the cation exchange step was deleted (13). The suggestion was made that better recovery might be obtained with the cation exchange resin in the sodium ion form. The “Initial Pretreatment” step was also discontinued for septic tank samples. ACKNOWLEDGMENT
The assistance of D. F. Kuemmel and T. R. Williams is gratefully acknowledged. RECEIVED for review July 30, 1970. Accepted September 28, 1970. (13) J. E. Thompson, Sanitary Engineering Research Laboratory, Procter & Gamble Co., Cincinnati, Ohio, personal communication, 1970.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 1, JANUARY 1971
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